fixed bug in entrypoint.sh

* use "hostname -I" instead of "hostname -i", as no DNS resolution is
  required

* "hostname -I" might return multiple IPs (as space delimited string).
  Use the first one ("best effort") per default. In case a different IP
  should be used for the swagger UI API resource, the "SWAGGER_IP" env
  variable must be set in the docker-compose.yml.

see #230

entrypoint.sh

@@ -29,7 +29,7 @@ service supervisor start
 supervisorctl start all
 fi
 
-export HOST_IP=$(hostname -i)
+export HOST_IP=$(hostname -I | awk '{print $1}')
 
 # Start API as signal-api user
 exec setpriv --reuid=${SIGNAL_CLI_UID} --regid=${SIGNAL_CLI_GID} --init-groups --inh-caps=$caps signal-cli-rest-api -signal-cli-config=${SIGNAL_CLI_CONFIG_DIR}